refactor: introduce constants for improved maintainability in tests and server configurations

- Added constants for test endpoints, usernames, and XML headers in client_test.go and device_certificates_test.go to enhance readability and reduce hardcoded values.
- Updated various test cases to utilize these constants, ensuring consistency across tests.
- Refactored imaging settings and server configurations to use defined constants for default values, improving clarity and maintainability in server/device.go and server/imaging.go.
- Enhanced comments throughout the code to clarify functionality and adhere to best practices.

server/imaging.go

@@ -377,12 +377,12 @@ func (s *Server) HandleGetOptions(body interface{}) (interface{}, error) {
 		},
 		WideDynamicRange: &WideDynamicRangeOptions{
 			Mode:  []string{"OFF", "ON"},
-			Level: &FloatRange{Min: 0, Max: 100},
+			Level: &FloatRange{Min: 0, Max: 100}, //nolint:mnd // Imaging parameter range
 		},
 		WhiteBalance: &WhiteBalanceOptions{
 			Mode:   []string{"AUTO", "MANUAL"},
-			YrGain: &FloatRange{Min: 0, Max: 255},
-			YbGain: &FloatRange{Min: 0, Max: 255},
+			YrGain: &FloatRange{Min: 0, Max: 255}, //nolint:mnd // White balance gain range
+			YbGain: &FloatRange{Min: 0, Max: 255}, //nolint:mnd // White balance gain range
 		},
 	}